Why do metal-poor stars have a wider range of orbital shapes than metal-rich stars like the sun?

The oldest stars are metal poor because they formed before there had been many supernova explosions to create and scatter metals into interstellar medium. Those stars formed long ago when the galaxy was young and motions were not organized into a disk, so the stars tended to take up randomly shaped orbits, many of which are quite elongated and eccentric. Consequently, today, the most metal poor stars tend to follow the most eccentric orbits.